Most of the time, the voice-grade telephone line cannot be used directly to send digital signals because its bandwidth is limited. However the line can be used to send digital signals if the signals are first converted into analog signals whose frequencies fit within the voice-grade line bandwidth. Such a conversion is carried out at a data communication adapter commonly known as a modulator-demodulator or a "modem" for short. A modem both converts digital data from a digital computing machine into an analog signal suitable for transmission over voice-grade telephone lines, and also converts a received analog signal into digital data for use by a digital computing machine. Among various modulation techniques one type of the Frequency Modulation (FM) known as Frequency Shift Keying (FSK) is very popular for modems operating at a medium or low speed, such as at bit rates less than 1800 bps (bits per second). In FSK, two different frequency tones are used to represent "zero" (or space) and "ones" (or mark) bits. When a "one" bit is sent to the modem, a low frequency tone is output to the telephone line. When a " zero" bit is sent to the modem, a high frequency tone is produced. The FSK signal is coherent, that is to say, the mark and space frequencies follow each other's phase continuously. Typically for the 1200 baud transmission, the mark frequency is 1200 Hz and the space frequency is 2200 Hz.
U.S. Pat. No. 4,568,882 (Single), issued Feb. 4, 1986, describes a digital FSK demodulator which uses modified mark and space filters to produce mark and space filter outputs. The outputs are rectified and compared with one another at a comparator to generate a baseband signal. U.S. Pat. No. 4,752,742 (Akaiwa), issued Jun. 21, 1988, teaches a FSK signal demodulator with a quadrature detector. The demodulator includes, in addition to the quadrature detector, a local oscillator and a logic circuit. The local oscillator has an oscillation frequency substantially equal to the centre frequency of the received signal to produce a baseband signal in a quadrature phase relationship. The baseband signal is produced as an output of the logic circuits, depending upon whether the received FSK signal frequency is higher or lower than the local oscillator frequency. U.S. Pat. No. 4,785,255 (Lucak et al), issued Nov. 15, 1988, mentions a digital FSK signal demodulator which contains a master clock for generating a plurality of pulses during each half of the bit interval of the FSK signal. A detector counts the number of master clock pulses during each bit interval to produce the baseband signal. These techniques perform well enough for demodulation of the FSK signals and they are essentially all digital in operation. However they all require a substantial amount of costly digital components.